diff --git a/src/arrays.test.ts b/src/arrays.test.ts
new file mode 100644
index 0000000..03aa9d2
--- /dev/null
+++ b/src/arrays.test.ts
@@ -0,0 +1,36 @@
+import { describe, expect, it } from "vitest";
+import { chunk, dedupe, groupBy } from "./arrays";
+
+describe("array helpers", () => {
+  it("splits arrays into fixed-size chunks and keeps a short tail", () => {
+    expect(chunk([1, 2, 3, 4, 5], 2)).toEqual([[1, 2], [3, 4], [5]]);
+  });
+
+  it("returns an empty array for non-positive chunk sizes", () => {
+    expect(chunk([1, 2, 3], 0)).toEqual([]);
+    expect(chunk([1, 2, 3], -1)).toEqual([]);
+  });
+
+  it("deduplicates values while preserving first-seen order", () => {
+    expect(dedupe(["ts", "js", "ts", "go", "js"])).toEqual(["ts", "js", "go"]);
+  });
+
+  it("groups items by the key returned from the callback", () => {
+    const grouped = groupBy(
+      [
+        { type: "fruit", name: "apple" },
+        { type: "veg", name: "carrot" },
+        { type: "fruit", name: "pear" },
+      ],
+      (item) => item.type,
+    );
+
+    expect(grouped).toEqual({
+      fruit: [
+        { type: "fruit", name: "apple" },
+        { type: "fruit", name: "pear" },
+      ],
+      veg: [{ type: "veg", name: "carrot" }],
+    });
+  });
+});
diff --git a/src/dates.test.ts b/src/dates.test.ts
new file mode 100644
index 0000000..913b509
--- /dev/null
+++ b/src/dates.test.ts
@@ -0,0 +1,29 @@
+import { describe, expect, it } from "vitest";
+import { addDays, daysBetween, isWeekend, startOfDayUTC } from "./dates";
+
+describe("date helpers", () => {
+  it("calculates absolute day differences", () => {
+    expect(daysBetween(new Date("2026-05-01T00:00:00Z"), new Date("2026-05-04T00:00:00Z"))).toBe(3);
+    expect(daysBetween(new Date("2026-05-04T00:00:00Z"), new Date("2026-05-01T00:00:00Z"))).toBe(3);
+  });
+
+  it("detects weekend days", () => {
+    expect(isWeekend(new Date("2026-05-30T12:00:00Z"))).toBe(true);
+    expect(isWeekend(new Date("2026-05-31T12:00:00Z"))).toBe(true);
+    expect(isWeekend(new Date("2026-05-29T12:00:00Z"))).toBe(false);
+  });
+
+  it("adds days without mutating the original date", () => {
+    const original = new Date("2026-05-29T10:30:00Z");
+    const result = addDays(original, 5);
+
+    expect(result.toISOString()).toBe("2026-06-03T10:30:00.000Z");
+    expect(original.toISOString()).toBe("2026-05-29T10:30:00.000Z");
+  });
+
+  it("returns the UTC start of the provided day", () => {
+    expect(startOfDayUTC(new Date("2026-05-29T23:59:59Z")).toISOString()).toBe(
+      "2026-05-29T00:00:00.000Z",
+    );
+  });
+});
diff --git a/src/money.test.ts b/src/money.test.ts
new file mode 100644
index 0000000..a81fa63
--- /dev/null
+++ b/src/money.test.ts
@@ -0,0 +1,27 @@
+import { describe, expect, it } from "vitest";
+import { formatCents, netPayoutCents, platformFeeCents, splitEvenly } from "./money";
+
+describe("money helpers", () => {
+  it("formats cents for USD, EUR, and fallback currency codes", () => {
+    expect(formatCents(12345)).toBe("$123.45");
+    expect(formatCents(987, "EUR")).toBe("€9.87");
+    expect(formatCents(500, "GBP")).toBe("GBP 5.00");
+  });
+
+  it("calculates platform fees in basis points and guards invalid inputs", () => {
+    expect(platformFeeCents(10_000, 250)).toBe(250);
+    expect(platformFeeCents(999, 333)).toBe(33);
+    expect(platformFeeCents(0, 250)).toBe(0);
+    expect(platformFeeCents(10_000, 0)).toBe(0);
+  });
+
+  it("subtracts platform fees from gross payout", () => {
+    expect(netPayoutCents(10_000, 250)).toBe(9_750);
+  });
+
+  it("splits cents evenly and distributes remainder to early recipients", () => {
+    expect(splitEvenly(10, 3)).toEqual([4, 3, 3]);
+    expect(splitEvenly(4, 2)).toEqual([2, 2]);
+    expect(splitEvenly(10, 0)).toEqual([]);
+  });
+});
diff --git a/src/strings.test.ts b/src/strings.test.ts
index daeb1cc..add3e00 100644
--- a/src/strings.test.ts
+++ b/src/strings.test.ts
@@ -1,7 +1,34 @@
-import { describe, it, expect } from "vitest";
-import { slugify } from "./strings";
-describe("slugify", () => {
+import { describe, expect, it } from "vitest";
+import { escapeHtml, slugify, titleCase, truncate } from "./strings";
+
+describe("string helpers", () => {
   it("lowercases and dashifies basic input", () => {
     expect(slugify("Hello World")).toBe("hello-world");
   });
+
+  it("removes punctuation and collapses repeated separators while slugifying", () => {
+    expect(slugify("  Hello,   Tiny---Utils!!  ")).toBe("hello-tiny-utils");
+  });
+
+  it("truncates long strings with the default suffix", () => {
+    expect(truncate("abcdef", 4)).toBe("abc…");
+  });
+
+  it("returns the original string when no truncation is needed", () => {
+    expect(truncate("abc", 4)).toBe("abc");
+  });
+
+  it("returns an empty string for non-positive truncation lengths", () => {
+    expect(truncate("abc", 0)).toBe("");
+  });
+
+  it("converts words to title case", () => {
+    expect(titleCase("hELLO tiny UTILS")).toBe("Hello Tiny Utils");
+  });
+
+  it("escapes HTML-sensitive characters", () => {
+    expect(escapeHtml(`<a href="x">Tom & 'Jerry'</a>`)).toBe(
+      "&lt;a href=&quot;x&quot;&gt;Tom &amp; &#39;Jerry&#39;&lt;/a&gt;",
+    );
+  });
 });